ADVANTAGES OF GNSS-BASED TERMINAL PROCEDURES

Italian Association of Aeronautics and Astronautics XXII Conference Napoli, 9-12 September 2013 ADVANTAGES OF GNSS-BASED TERMINAL PROCEDURES G.B. Palmerini *, M. Simone, A. Simone DIAEE - Dipartimento di Ingegneria Astronautica, Elettrica ed Energetica, Sapienza Università di Roma, via Salaria 851 00138 Roma, Italy *giovanni.palmerini@uniroma1.it ABSTRACT The air traffic control scenario shows a significant trend towards the introduction of new procedures based on global navigation satellite systems (shortly GNSS) capabilities. This trend, already successful in the U.S., is spreading now in Europe, also due to the achieved operational capability of the EGNOS system, and is expected to further increase with the future achievement of Galileo s operational capability. The expected advantages at the network level are quite remarkable, with the reduction of ground-based infrastructures and the (although far in time) standardization on a single air navigation aiding system. The adoption of GNSS, useful during each and every step of the flight (take-off, en-route, approach, landing and even taxiing) will provide substantial benefits for on-board equipment, maintenance, crew s training. This path is clear and widely accepted, but several aspects still have a significant interest for researchers, above all with respect to the final flight phases (approach and landing), that are also the most challenging from the technical point of view. Main issues deal with the evaluation of the satellite-based procedures impact in an operational scenario. The newly designed approach procedures can be easily simulated to provide a detailed analysis of the trajectory of the aircraft. With respect to the obtained position/time history, it is then possible to evaluate their performance in terms of air traffic capabilities compared to traditional ILS- or NDB-based procedures. The paper discusses the case for Parma airport by comparing conventional and satellite-based procedures, and details the advantages of the latter in terms of operations as well as in terms of environmental impact (noise) and operative cost for the planes owners are considered. Keywords: Navigation Aids, GNSS, Approach Procedures, Airport Capacity. 1 INTRODUCTION The average increment - on a several years basis - of the air traffic calls for an in-depth revision and improvement of the network procedures, especially in crowded areas as mainland U.S. and Western Europe. It has been foreseen since long time that such an improvement could be granted by the introduction of procedures based on satellite navigation systems, instead of ground-based traditional radio navigation aids. The proposed evolution, aimed to increase the capabilities of the air traffic system (reduced in-flight separation, RNAV) was also strongly backed by economic aspects, with the outlook of the substitution of a host of different systems with a single air navigation equipment, possibly suitable for all phases of 1

flight and in each environment of interest. This extremely significant transition has been technically studied in depth since the advent of the first modern, operational GNSS (the GPS- NAVSTAR), showing as an example the technical feasibility of CAT III approaches and landings ([1],[2]). With the addition of other GNSS (Glonass, the incoming Galileo) and of the satellite based augmentation systems (SBAS, as WAAS, EGNOS, MSAS, GAGAN, QZSS) the rich set of available signals and services pushed towards worldwide application of the new navigation method. The interest slowly shifted from a purely technical point of view to an operational one, aimed to evaluate and validate the performance of satellite-based techniques in the frame of the real-world aviation, with strict safety rules and certification issues. The more challenging phases are clearly the final ones, namely approach and landing, where the integrity of the GNSS signal, in addition to its availability and accuracy, can be a problem [3]. Full operational capability of SBAS and spreading of ground based augmentation systems (GBAS, [4]) contribute to the solution, paving the way to the definition of a number of flight procedures based on satellite navigation systems enabling the approach to many airports. With the fielding of the different components (in space, onboard, on the ground) of this system of systems devoted to air navigation there is now the chance to evaluate its operational benefits with respect to traditional rules. This paper builds such an analysis for the approach phase (i.e. the more interesting one as considered the bottleneck for the air traffic). Specifically, the availability of satellite-based experimental approach procedures for the Parma and Perugia airports allowed to a comprehensive comparison with the conventional, already available procedures [5]. This paper collects the main findings of that work, presenting the case for Parma and detailing the advantages of GNSS approaches with respect to existing NDB and ILS ones. The identified benefits include the increase in the airfield capability as well as the reduction in the air traffic environmental impact and the savings in aircraft operator s costs. To compute these benefits the procedures for Parma are first presented (section 2). Next, the effects strictly related to the allowed air traffic capacity are discussed (section 3), as well as the impact on the noise levels and on the operation costs (section 4). After the final remarks and the acknowledgements, a short reminder of the acronyms is included. 2 PROCEDURES DESIGNED FOR PARMA AIRPORT The comparison between traditional and GNSS-based procedures will be carried out with respect to Parma airport, a moderate traffic site (25 serviced flights per day in 2011/2012), located at a short distance (1.5 NM) from city center. The airport is provided with a single runway (02/20), 2124x45, with only threshold 20 enabled for instrumental approaches, while threshold 02 can be used for visual approaches following the authorization from the tower. The entrance from en-route phase to STAR (Standard Instrumental Arrival) can be either from North-West significant point MISPO - or South-East gate significant point LUPOS (Fig.1). Instrumental approach procedures include NDB (NPA or Non-Precision Approach) or ILS (PA or Precision Approach, actually available as Z, the one referred to in the following, and Y) procedures, both with only one aircraft allowed at the time. NDB RWY 20 and ILS-Z RWY 20 are almost equivalent, meaning that they are designed on the same significant points with the first based on time intervals and the second on the distances from DME. These procedures foreseen an inbound flight towards PAR NDB (located on the airfield) then a reverse, with its relevant turns, to be again on the fix and become the final phase at 3000 ft. Incoming traffic from LUPOS, or traffic from MISPO when the path is not clear should enter the holding circuit at 4000 ft in PAR NDB and wait for the preceding aircraft to land. 2

Figure 1: STAR chart for Parma airport, showing waypoint MISPO, beginning gate for the investigated procedures as well as the alternate SE gate LUPOS [Reported from ENAV AIP Italia. Disclaimer: the present chart, as well as the ones in Figure 2, are reproduced only to help in the discussion proposed in the paper] The main concern for the approach to Parma is due to the limited and constrained airspace, which is bounded by close CTRs (Verona, Piacenza and Bologna) and military segregated areas, leading pilots to mandatory and frequent shifts in altitude, steep turns and high descent rates. In addition, the missed approach procedure dictates a left turn above the city center in a steep and continuous raise to re-gain PAR NDB. Figure 2 (upper plot) reports the ICAO chart for the ILS approach (being the NDB one quite similar). The overall time interval between landings (landing rate) for these procedures amounts to 7 minutes. Following the GNSS systems spreading, and the interest for this appealing and innovative technique, a number of satellite based procedures have been designed for several Italian airports. Among them, two GNSS/EGNOS SBAS procedures have been prepared for Parma airport threshold 20 (NPA and APV I/II), and, additionally, an instrumental GNSS-based procedure has been prepared for threshold 02, previously lacking an instrumental approach capability (see [6] for APV definitions). All of these procedures are currently - experimental and non-operational. In the following we will refer to threshold 20 procedures because they allow a comparison with already existing conventional ones. GNSS/EGNOS SBAS NPA and APV I/II have been designed with the goal to limit the flight phases above city center, to extend the operations in Parma to D-type aircraft (see next paragraph), to clearly split standard approaches from missed ones, to limit flight paths, noise 3

Figure 2: ILS (top) and GNSS (bottom) procedures for the approach and landing in Parma, runway 20 [ILS chart from ENAV AIP Italia, GNSS chart courtesy of IDS / SIAM Project] 4

and pollution. The solution to these requirements has been found in a straight-in approach that collects the incoming traffic at MISPO and, through a preparatory turn, aligns it with threshold 20 without the useless leg to the airfield vertical aimed to overfly the NDB to fix and the following reversal. The resulting path is smoother, without levelling phases (Figure 2, bottom plot). Also, the missed approach path is far easier for the pilot, with a moderate continuous climb to enter the holding circuit SE of the airfield, instead of the climb-turnclimb to gain the holding circuit above city center foreseen in traditional procedures. 3 THE EFFECTS ON THE AIRPORT CAPACITY The new approach procedures based on GNSS affect the air traffic handled by Parma airport in three different ways: - with respect to the trajectories followed by planes arriving from NW (MISPO); - with respect to the time each single landing plane requires from the approach gate to the touch down; - with respect to the along-path altitude clearances, affecting the visibility (i.e. weather) conditions in which operations on runway 20 are allowed. The first two aspects, which are strictly related, have been the object of a quantitative analysis [5] based on the ICAO rules, on the procedures description and on the airport Instrumental Approach Charts reported in Figure 2. By considering the aircraft partition according to ICAO [6], reported in Table 1 together with some example of well-known plane for each category, it is possible first to assess the expected characteristics for every traffic component. Table 2 reports these characteristics in terms of the speed maintained along different approach segments for the considered aircraft categories. Category A B C D Aircraft General aviation (tipically single engine / propeller) AC11 Commander, PA23 Apache,PC-12 Pilatus,C150 Commuter General aviation (twin engine / propeller) ATR42, BE20 Superking, F27 Fokker, FA20 Falcon 20, P180 Piaggio Commercial trurboprop and small jetliners, executive jets F28, C650 Citation,GLF3 Gulstream III, IL18 Ilyushin,B737 Boeing Commercial jetliners A330-340 Airbus, B747 -B767 Boeing, TU134 Tupolev Table 1: Aircraft partition in categories Speed at threshold Range of speeds for initial approach Range of speeds for final approach Max speeds for intermediate missed approach A <91 90/150 (110*) 70/100 100 110 Aircraft Category Max speeds for final missed approach B 91/120 120/180 (140*) 85/130 130 150 C 121/140 160/240 115/160 160 240 D 141/165 185/250 130/185 185 265 Table 2: Characteristic speed (Kt) for different aircraft categories at various approach steps (* indicates maximum speed for reversal procedure) 5

By means of these data and of the procedures description it is possible to compute the time required to fly each approach segment of the procedures and the relevant length, taking into account possible constraints on the manoeuvres like bank angle limits in turns. Table 3 reports this evaluation for the conventional approach procedure beginning at the NW gate (MISPO) carried on by means of the ILS (precision approach) in the specific case of a plane belonging to category C. The two cases of an approach successfully ending with the plane at the runway threshold, close to touch down, or of a missed approach call, with the following climb and the insertion in a holding circuit are reported. Representative waypoints along the similar NDB procedure are reported in Figure 3, which is referred to A-type aircraft. approach segment segment length (NM) allowed IAS (Kt) constraints on IAS / bank angle flight time (mm:ss) MISPO - IAF (STAR) 15 160/240-05:37/03:45 IAF FAF 6.5 + 4 + 2.3 = IAS 140 / 15 160/240 (reversal) 12.8 (on turn) 05:29 FAF MAPt 3 115/160-01:33/01:07 MAPt THR 0.4 115/160-00:12/00:09 Totale (MISPO- 31.2 12:51 / 10:30 Threshold) THR MA TP 5.1 160-01:54 MA Turn 8.1 240 IAS 185/15 (on turn) 02:37 MA from Turn to Holding 4.4 240-01:06 Total 48.8 Total 18:28 / 16:07 Table 3: Length and related flight time for conventional procedure precision approach exploiting ILS. Values refer to paths from MISPO to threshold and from MISPO to hold circuit (missed approach) Approach segment Distance (NM) Segment # Figure 3: The sequence of waypoints along the conventional NDB procedure for the approach and landing in Parma, runway 20. Path refers to A-type aircraft 6

Of course the same computation can be performed for different procedures and different aircraft categories (to be noticed that D category operations are not foreseen in Parma within current procedures). Table 4 reports a similar computation, still for C-type planes, relevant to the newly designed satellite-based procedure for precision approach. The location of the waypoints, different from the conventional procedure, can be easily gained from the relevant chart in Figure 2 (bottom plot). This different approach clearly leads to a shorter path, both in distance and in time. approach segment MISPO - IAF (STAR) IAF FAF segment length (NM) allowed IAS (Kt) Constraints on IAS / bank angle flight time (mm:ss) 3 160/240-01:07/00:45 8 + 1.9 +2.7 = IAS 185 / 25 160/240 (reversal) 12.6 (on turn) 04:43/04:05 FAF MAPt 3.3 115/160-01:43/01:14 MAPt THR 0.8 115/160-00:25/00:18 Total (MISPO- Threshold) 19.7 07:58 / 06:22 THR MA TP 6.5 160-02:26 MA Turn 2.3 240 IAS 185 / 15 (on turn) 00:44 MA from Turn 6.1 240-01:32 to Holding Total 34.6 Total 12:39 / 11:04 Table 4: Length and related flight time for satellite-based procedure precision approach interesting C-type aircraft. The values are related to the paths from MISPO to threshold and from MISPO to holding circuit beginning (in case of missed approach) By carrying out the computation for every plane category and along all possible (conventional and satellite-based) approach paths, it is possible to obtain a global insight on the way the procedures affect the inbound air traffic at Parma airport. The following Figure 4 depicts this comparison in distance and in flight time between NDB and GNSS non-precision procedures for runway 20, showing a distinctive advantage in efficiency for the satellite-based technique. Figure 4: Instrumental, non-precision approaches: comparison between conventional NDB (blue) and GNSS-based (red) procedures 7

The landing rate currently enabled by conventional (NDB or ILS) procedures is about 9 aircraft/hour, with a 7 minutes interval between two planes following each other. With the introduction of the GNSS-based procedures, the previous analysis suggests the possibility leads to limit the time spacing between following planes to 4 20, reaching the 14 aircraft / hours landing rate. These data, also reported in Table 5, have been analysed in depth trough the preparation and the exploiting of a sequencer, a powerful tool to represent and organize in a correct schedule the sequence of approaches. Findings previously reported have been validated as significant by means of this purposely designed spread-sheet. The tests adopted a reasonable mix of incoming aircraft in terms of type composition for Parma airport expected traffic, and Monte-carlo runs to analyse different sequences. Procedure (max) Arrival / h Departure+Arrival / h Conventional (ILS Cat.1) 9 18 GNSS-based APV 15 30 Mixed (ILS+GNSS) 11 22 Table 5: Aircraft capacity enabled by different procedures The improvement with respect to current status (ILS) is in the order of 22% if only a part of the incoming aircraft can follow satellite procedures, while attains the 68% if all the arriving aircraft are equipped with GNSS equipment. The sequencer always reaches the goal to double the number of served aircraft by successfully interleaving a take-off between two landings. Finally, there are the advantages coming from the innovative design of the GNSS procedures - based on virtual waypoints instead of on ground-related fixes - that allows for a more flexible altitude separation from ground along the approach path. Such a condition, leading to different values for the OCA (Obstacle Clearance Avoidance, computed from the mean sea level) and OCH (Obstacle Clearance Height, reckoned from the ground), is extremely important, above all with an airfield with typical fog condition as Parma. Table 6 shows that satellite-based procedures provide a distinctive advantage, allowing the planes to fly lower with respect to conventional approaches. Poorer visibility conditions can be therefore accepted, ending up with an increased time-availability of the airfield. Procedure OCA (OCH) A B C D ILS RWY20 340(195) 345(200) 350(205) - Approach GNSS APV I RWY20 GNSS APV II RWY20 420(275) 430(285) 440(295) 450(305) 395(250) 410(265) 415(270) 425(280) Comparison OCA/H (ILS-APV I RWY20) Comparison OCA/H (ILS-APV II RWY20) 80 85 90 * 55 65 65 * Table 6: Comparison between conventional and satellite-based precision approach procedures in terms of OCA and OCH (data in ft; the symbol * is a reminder that D-type aircraft are not considered in conventional procedures) 8

4 ADDITIONAL EFFECTS OF GNSS BASED PROCEDURES 4.1 Quantifiable and non-quantifiable benefits Previous section proved an advantage of the GNSS-based procedures in terms of airport traffic capability. While that performance has been clearly assessed by numeric figures, it is not always possible to correctly quantify other benefits, either because they are in nature difficult to compute or because they depend on a large and ambiguous number of variables that makes their computation ineffective. Among them there are, as an example, the reduction in the crew s workload due to faster and smoother procedures or the increase in safety due to this easier workload as well as, from a different point of view, to the reduction in city center overflying. There are however two other effects that are suitable to be quantified, and which will be shortly described in the following (see [5] for additional details). 4.2 Acoustic noise A relevant factor to be considered while evaluating the approach procedures is their noise impact. Airports are increasingly challenged on this issue due to environmental concern, and the impact of the approaching path in terms of noise became a serious show-stopper in the development of new airfield or runway addition. In the case of Parma airport the current status is clearly represented by the isophonic curves envelope reported in Figure 5. Their overlapping on the urban area is produced by the flyby for the fix above NDB PAR at 2500 ft, and the following reversal to align to the runway threshold 20, an unfortunate result of the conventional procedures. It is worth to mention that also the holding circle in case of busy runway or missed approach lies above the city. Instead, satellite-based procedures move away the initial holding circuit as well as the one following the missed approach, as it can be seen in Figure 6. Specifically the initial holding is located north-west, above the banks of the Po River, and the closest urban area ends up to be the town of Busseto, 10 km away from the centroid of the noise envelope. Even better, the holding following a missed approach is moved over the hills south-east of the city, i.e. above uninhabited areas. Relevant isophonic curves do not even overlap with current, problematic ones represented in Figure 5. Figure 5: Isophonic curves relevant to current NDB-based procedure 9

Figure 6: Holding areas for NDB (conventional procedure, above Parma city center, in cyan) and GNSS-based (green - initial, and yellow in case of missed approach) procedures, showing the latter one location far from the airport and urban area 4.3 Operating costs The analysis in [5] also proved the economic advantages that GNSS-based procedures have for the operators. By selecting a test case based on a C-type aircraft (the executive Learjet 24), and referring to existing cost databases, it is possible to define the total flight-hour cost as the combination of the recurring and fixed expenses and overheads. This flight-hour cost (about 4000 US $ at 2013 values) can be reported to the duration of the approach procedures to approximately identify the relevant cost of the single flight event. The reduction in the approach manoeuvre cost amounts to about 40% in case of precision approaches (i.e. GNSS SBAS APV I/II with respect to conventional ILS) and even slightly more in case of nonprecision approaches (GNASS SBAS vs. NDB). Furthermore, these savings can be directly compared to the overall flight-hour figure. The resulting 10% approximate reduction in a flight hours including a landing suggests a serious benefit in case of a 1-2 hours flight duty cycle of the aircraft, in some way typical for European users. 5 CONCLUDING REMARKS The introduction of GNSS-based arrival procedures is the more challenging step in the incoming and widespread move towards a satellite-based air navigation system. Aside from the already assessed - performance of the new techniques in theory, there is still a strong interest to check their viability in a real operation scenario, including typical aeronautical rules and certification issues. By means of the comparison between experimental GNSS procedures and conventional NDB and ILS ones, all of them referred to threshold 20 at Parma airport, this paper evaluates the benefit in terms of air traffic capacity handled by this single runway system. The advantages, validated through a purposely built scheduler for the sequence of operations on a representative sample of incoming air traffic, are clear, with a reduction in the approach time and an increase in the number of take-offs and landings. The design of GNSS approaches, built on virtual waypoints, allows more efficient paths, saves from the over-flight of the city center and accepts lower altitude minima, also increasing the availability with respect to foggy weather conditions. Even more important, GNSS-based procedures end up to be smoother, limiting the needs of turns, reducing the pilots workload and therefore increasing safety. 10

Also, GNSS-based procedures provide significant value with respect to the noise impact, an issue of ever increasing relevance. In fact, newly designed holding, not anchored to groundfixed beacons, can be moved away from the airfield, dropping the environmental impact. In addition, the possibility of a limited but interesting economic benefit for the aircraft operators, mainly due to the shortest flight to reach the airfield has been proven. Quite interestingly, similar results have been obtained in [5] also for Perugia airport, showing that benefits from the introduction of satellite-based navigation procedures in approach phases can be roughly deemed as general and not limited to single, special cases. 6 ACKNOWLEDGEMENTS A large part of the accompanying material originates from the unpublished referenced work [5]. The authors acknowledge the contribution of ENAV and IDS in making this material available for research purposes as well as the kind and fruitful help from their extremely skilled personnel. 7 ACRONYMS APV CTR EGNOS FAF GAGAN GBAS GLONASS GNSS GPS IAF ILS MA(P) MSAS NDB QZSS STAR WAAS REFERENCES Approach procedure with Vertical Guidance Control Tower European Geostationary Navigation Overlay System (E.U. SBAS) Final Approach Fix GPS-Aided Geo-Augmented Navigation (Indian SBAS) Ground-Based Augmentation System Global Navigation Satellite System (Russian System) Global Navigation Satellite System Global Positioning System (U.S. System) Initial approach Fix Instrumental Landing System Missed Approach (Point) Multi-functional Satellite Augmentation System (Japanese SBAS) Non Directional Beacon Quasi-Zenith Satellite System (2 nd generation Japanese SBAS) Standard Terminal Arrival Route Wide Area Augmentation System (U.S. SBAS) [1] B.W.Parkinson, M.L.O Connor, K.T.Fitzgibbon, Aircraft Automatic Approach and Landing Using GPS, in (Parkinson, Spilker, Axelrad, Enge Eds.) Global Positioning System: Theory and Applications, AIAA, Washington (USA), Vol.II, pp.397-426 (1996). [2] C.E.Cohen, B.S.Pervan, H.S.Cobb, D.G.Lawrence, J.D.Powell, B.W.Parkinson, Precision Landing of Aircraft Using Integrity Beacons, in (Parkinson, Spilker, Axelrad, Enge Eds.) Global Positioning System: Theory and Applications, AIAA, Washington D.C. (USA),Vol.II, pp.427-459 (1996). [3] P.D. Tromboni, G.B. Palmerini, "Navigation Aids Performance Evaluation for Precision Approaches", International Journal of Aerospace Engineering, vol. 2010, Article ID 389832, pp.1-10 (2010). [4] P.D. Tromboni, G.B. Palmerini, P. Gervasoni Experiences in Data Analysis of a GBAS Test, paper IEEEAC1654, IEEE Aerospace Conference Proceedings (2007). [5] M. Simone, Analisi delle capacità aeroportuali con procedure di volo convenzionali e satellitari, Tesi di laurea in Ingegneria Aerospaziale, Sapienza Università di Roma (2013). [6] ICAO - DOC 8168 OPS/611, Vol.II, 5th Ed. (2006). 11